Electromagnetic brakes are known that comprise a stack of disks, one portion of which (rotor portion) is constrained to rotate with the wheel, and another portion of which (stator portion) is prevented from rotating, together with one or more electromechanical actuators, each of which has a pusher for applying pressure to the stack of disks in controlled manner.
For example, reference can be made to Documents FR-A-2 824 967, U.S. Pat. No. 6,189,661, and U.S. Pat. No. 4,809,824.
Such a brake is, in general, equipped with a parking device organized to maintain pressure on the disks at least while the brake is not being powered electrically. The parking device serves to hold the vehicle stationary when it is stopped.
Parking devices for electromechanical brakes are known that comprise controlled means for positively blocking at least one of the actuators equipping the brake so that the pusher of said brake is held stationary in a position in which it is pressed against the stack of disks.
However, such parking devices are unsatisfactory. After intense braking, the component parts of the brake (stack of disks, brake structure, and actuators), which have been subjected to considerable amounts of heat, are in an expanded state. As a result, when the blocking means of the parking device are actuated soon after intense braking, they block the pusher while the brake is in an expanded state.
The coefficient of expansion of the disks is lower than the coefficient of expansion of the metal portions of the brake and of the wheel. The structure of the brake and the blocked pushers form a clamp that clamps the stack of disks, which stack is then progressively compressed during cooling.
That compression might damage the disks and might block the pusher so strongly so that it is then difficult or even impossible to release it.
In order to eliminate that drawback, consideration has been given to modifying the parking device so as to equip it with a sliding member that makes it possible to limit the pressure force delivered by the pusher against the stack of disks to a predetermined value, so that it is possible to cause the pusher to be blocked without any risk, even while the brake is still in the expanded state, the extra force on the stack of disks that is generated by the brake cooling then being absorbed by the sliding member.
It has also been proposed to use resilient means for maintaining a parking force in spite of the thermal expansion of the brake, e.g. Belleville spring washers which are compressed under drive from the motor once the pusher has come into contact with the disks. The compression is maintained because of the irreversibility of the coupling between the motor and the pusher.
But, when the outside temperature increases (e.g. when going from night to day for an aircraft parked on an airport in a desert region), the brake expands again, the faster expansion of the metal portions of the brake and of the wheel causing a reduction in the pressure force, which is undesirable insofar as that force could become insufficient to hold the vehicle stationary on a slope.